At the same time, it's important to avoid blocking the water supply- if anything ever goes wrong with the sink or the dishwasher, you have to be able to get in there and shut it off. This shelf slots into place, so that you can pull it out at a moment's notice.
The shelf that I built fits into the corner, taking advantage of the otherwise dead space beside the disposal. It rests on brackets on one side, and slots between upper and lower brackets in the back. It can hold a surprising amount of weight. The dishwasher hose loops around it, and it doesn't block the shutoffs. If anything ever needs to be shut off, disconnected, or re-connected in a hurry, all you have to do is pull the shelf straight out.
Step 1: Tools and Materials
1. A drill and bits
2. A screwdriver
3. A pencil
4. A level that is small enough to fit in under your sink
5. A tape measure
6. A straight-edge
7. Saw to cut the board to size (not shown, since the board I had was coincidentally a good size already)
Materials you will need:
A. A board to make the shelf out of
B. Angle brackets (6)
C. #6 screws (8)
Because you will be working in an area that has lots of water and electrical hook-ups and you can't be quite sure where they run, I recommend setting the bit on your drill quite short, and plugging it into a GFCI outlet if at all possible.
Step 2: Measuring
In my case, I had a re-used particleboard shelf that was a nice width already. All I had to do was figure out how long to make it. There was already a paper towel holder in the cabinet, so I cut the shelf short enough to leave an inch clearance between it and a full roll of paper towels.
Step 3: Making Guidelines
As it turned out, I had to re-do this step. The floors in my house are nothing like level, so the little shelf, level with the floor, turned out to be much too tilted to use. That was where the spirit level came into it- I re-did the lines and starter holes based on actual horizontality, rather than my home's bizarre opinion. Happily, it's under the sink; no-one will notice. I suggest that you make the shelf truly level as well- since it's only bounded on one side, things will roll off easily if it's crooked.
Step 4: Install the Brackets and Shelf
I'm sorry I don't have a good picture of the final step; the presence of the shelf pretty much hides the brackets.